1. Field of the Invention
The invention concerns the production of synthesis gas by a sequence of one or more endothermic and/or adiabatic steam reforming and autothermal steam reforming.
2. Description of the Related Art
U.S. Pat. No. 6,375,916 discloses a method for preparing synthesis gas by installing a pre-reformer upstream an autothermal reformer (ATR). The pre-reformer is used to remove or reduce the contents of higher hydrocarbons from a hydrocarbon feed stream with the advantage that lower steam to carbon ratios can be employed without soot formation in the ATR. However, the process described is not able to produce a synthesis gas with an hydrogen to carbon monoxide ratio close to 2.0 unless either the steam-to-carbon ratio is very low (probably less than 0.2) or the difference between the exit temperature from and the inlet temperature to the ATR is very high. In the former case this may give difficulties with operating the prereformer without carbon formation and in the latter case the amount of oxygen used may be disadvantageously high.
U.S. Patent Application Publication No. 2001/0051662 by Arcuri et al. discloses a method to produce synthesis gas involving among others the mixing of tail gas and a hydrocarbon feedstock and feeding the resultant mixture to an adiabatic pre-reformer. The effluent from the adiabatic pre-reformer is passed to a synthesis gas generator for production of synthesis gas.
If the synthesis gas generator is an autothermal reformer, a synthesis gas with a hydrogen to carbon monoxide ratio of about 2.0 can be produced. However, recirculation of the tail gas to the feed to the adiabatic pre-reformer is disadvantageous because the risk of carbon formation will be higher in the prereformer. This means that the process must be operated at a higher steam-to-carbon ratio. Low steam-to-carbon ratios are generally preferable in Fischer-Tropsch to improve economics.
U.S. Pat. No. 6,525,104 describes a process in which a heat exchange reformer is placed in series with and upstream of an Autothermal Reformer for production of synthesis gas. Recirculated carbon dioxide is added to the feed stream to the heat exchange reformer. The amount of recirculated carbon dioxide is adjusted to between 20 and 60% of the carbon from hydrocarbons in the feed stream to the plant. No prereformer is used. The carbon dioxide is recovered and recirculated from one of several possible locations downstream the Autothermal Reformer.
This concept has several disadvantages for production of synthesis gas for Fischer-Tropsch processes. One disadvatage is that a costly step of separating carbon dioxide from a mixed gas stream is needed. Another disadvantage is that it may not be possible with the amount of recirculated carbon dioxide to produce a synthesis gas with the desired hydrogen-to-carbon monoxide ratio (i.e. a H2/CO ratio of approximately 2.00) except possibly at relatively high steam-to-carbon ratios. In the examples given in U.S. Pat. No. 6,525,104 a steam-to-carbon ratio of 1.5 is used. A steam-to-carbon ratio of 1.5 will in many cases render a process for production of Fischer-Tropsch products uneconomical.
In another embodiment disclosed in U.S. Pat. No. 6,525,104 a higher hydrocarbon (hydrocarbons with two or more carbon atoms) and carbon dioxide containing gas stream is recirculated to the feed to an adiabatic prereformer placed upstream and in series with the heat exchange reformer and the autothermal reformer. If this recirculated gas stream is a tail gas from a Fischer Tropsch synthesis section, then this process would have the disadvantage of an increased risk of carbon formation in the prereformer as described above. Hence, a higher steam-to-carbon ratio would be needed. This may appear surprising as it is generally accepted that passing higher hydrocarbon containing gas streams through an adiabatic prereformer is advantageous from a process economic point of view.